Electronic paper, also called e-paper or electronic ink, are a range of display technology, which are designed to mimic the appearance of ordinary ink on paper, but that can be written to and erased electronically. Applications of electronic paper include electronic pricing labels, time tables at bus station, mobile phone displays and e-readers able to display digital versions of books and electronic paper magazines.
Electronic paper forms visible images by rearranging charged colored particles using an applied electric field. For example, Comiskey et al. Nature 1998, 394, 253 to 255 describes a display technology called vertical electrophoretic display (EPD). In one embodiment, this technology uses microcapsules having a transparent shell and filled with numerous slightly negatively charged white titanium dioxide microparticles dispersed in a dyed (Oil Blue N) dielectric fluid. The microcapsules are dispersed in a carrier (UV curable urethane) and subsequently coated onto a transparent conductive film (indium tin oxide on polyester). Rear electrodes printed from a silver doped polymeric ink are then applied to the display layer. Applying a positive charge to one or more rear electrodes results in the migration of the slightly negatively charged white titanium dioxide microparticles to the bottom of the local microcapsule, forcing the dyed dielectric fluid to the surface and giving the pixel a black appearance. Reversing the voltage has the opposite effect.
Whereas there are currently black and white electronic papers which mimic the appearance of ordinary ink on paper sufficiently, the development of full-colour electronic papers resembling coloured ink on ordinary paper is still an area of intense research.
Full-colour electronic paper can be generated a) by modulating light in an additive system with the primaries of red, green and blue (RGB-technology), b) by using s substractive system with cyan, magenta and yellow (CMY-technology) or c) by using a substractive/additive hybrid system using both RGB and CMY primaries in a cooperative “biprimary system” (J. Heikenfeld et al. Journal of the SID, 2011, 19/2, 129 to 156).
Both technologies (RGB-technology or CMY technology) require a dispersion of charged coloured particles in a dielectric fluid, wherein the charged particles show a narrow size distribution and thus form homogeneous dispersions.
When using the CMY technology, for example, the charged coloured particles should have a size in the range of about 1 nm to 100 nm. When using CMY technology in video applications, it is desirable to use charged coloured particles of the the smallest particle size possible, as a decrease in particle size yields an increase in the switching frequency of the images.
WO 2007/147742 describes a coloured compound of general formula
wherein    each of A and A′ is, independently of the other, C1-C4 alkyl;    CAGE is a moiety of the formula IA
wherein the asterisks (*) mark the bonds binding the moieties of the formula,
shown above, respectively,    D is a chromophoric moiety, with the proviso that all 8 moieties D in a molecule of the formula I are identical;    E is **—C(R3a)(R3)—C(H)(R3b)—** and/or
wherein the double asterisks (**) mark the binding bonds, respectively, and wherein each of R3, R3a and R3b, independently of the others, is hydrogen or unsubstituted or substituted C1-C12alkyl; L is unsubstituted or substituted C1-C25alkylene which is linear or branched, which alkylene may be bound* and/or be interrupted by at least one of the radicals selected from the group consisting of —O—, —S—, —N(R4)—, —CO—, —O—CO—, —CO—O—, —N(R4)—CO—, —CO—N(R4)— and phenylene, wherein R4 is hydrogen or unsubstituted or substituted C1-C12alkyl;    X is —NR5— or —O—; and    R5 is hydrogen or unsubstituted or substituted C1-C12alkyl;    or a salt thereof.
The coloured silsesquisiloxanes are used as colorants, pigments and dyes.
WO2007/048721 describes the use of functional particles as electrophoretic displaying particles, wherein the functionalized particles are SiO2, Al2O3 or mixed SiO2 and Al2O3 particles comprising covalently bound to an oxygen atom on the surface, a radical of formula (1),
wherein R1 and R2 are independently of each other hydrogen, particle surface —O—, or a substituent, n is 1, 2, 3, 4, 5, 6, 7 or 8, B is the direct bond or a bridge member, and D is the residue of an organic chromophore.
WO 2010/149505 describes a composition comprising charged particles, preferably having an inorganic core of SiO2, Al2O3 and/or TiO2, and a counter ion comprising a silicon atom which is directly bound to a carbon atom. Said composition may be used in electrophoretic displays. Preferably, said charged particles comprise a dye attached to said inorganic core and said counter ion comprise a (poly)siloxane moiety linked via suitable bridge members to a quaternary, positively charged nitrogen or phosphorus atom, or to a moiety carrying an anionic functional group. Exemplified are charged particles having a SiO2 core.